This invention relates to a method for inoculating liquid metal cast under low pressure.
As is known, inoculation of a laminated, spheroidal or vermicular graphite casting can be achieved using an inoculant such as ferrosilicon powder put into a mold beforehand, where the liquid casting metal is forced into a mold uphill by a relatively low gas pressure on the order of 0.2 to 1.5 bar.
It has generally been known for some time that the purpose of inoculating the casting metal using ferrosilicon or other graphitizing products is to promote graphitization, or the formation of free graphite during the solidification of the casting in order to obtain a good resilience of the cast product. This inoculation is more effective the closer it is done to the mold, just before the casting metal enters the mold. The effect of inoculating the liquid casting is short lived, and tends to decline after a few minutes. This makes it necessary to avoid too long a delay between the inoculation and charging the mold with the liquid metal.
There is also another conventional inoculation procedure using an endless wire inoculant. The wire is easy to handle mechanically by being unwound from a spool, ensures a precise dosage of the inoculant material, and easily melts in the liquid casting metal.
French Pat. No. 2,276,124 discloses a procedure for adding a reactive metal as an elongated element suspended inside a mold, which is filled with the liquid metal to be treated. When the level of the liquid metal rises in the mold, the extended reactive element melts in the molten metal, thus releasing the reactive metal in the molten mass. The mold is gravity-fed with liquid metal. Casting is thus downhill. No problem arises in suspending the extended reactive element inside the mold. This is done manually, and the inoculation takes place inside the mold.
French Pat. No. 2,278,432 involves the use of an inoculant in the form of an endless wire unrolled from a spool, to be introduced in vertical suspension into a basin provided in the mold. This basin is located in the path of the liquid metal being treated, such path going through a runner between the vertical casting gate and the casting hollow or mold. Due to the speed at which the liquid metal goes through this intermediate inoculation basin, in which the lower end of the inoculant wire is suspended, it is difficult to achieve a good homogeneity of liquid metal inoculation, and thus of the metal mass admitted into the mold. This risk of insufficient uniformity of inoculation is greater when the casting impression is larger or more complex, notably when casting thin pieces. In fact, the solidification of the liquid metal is so fast that it is completed before the inoculant wire can completely dissolve in the liquid metal, making the inoculation incomplete and nonuniform.
The document Giesserei-Praxis No. 3 of Feb. 10, 1982, pages 29-36, explains another technique for inoculating the casting by means of a wire unrolled from a spool. The wire is introduced into the center line of the gravity gate coming from a pouring basin or a stopper rod casting-ladle. Better inoculation uniformity results because the liquid metal remains in contact with the inoculant wire over a certain length of it, just before introduction in the mold's vertical gate.
These three conventional examples involve a vertical, gravity fed cast gate, however, rather than a vertical, uphill cast gate under low pressure to charge a mold.
There is thus a problem in inoculating by means of a wire in the technique of uphill casting of the pig under low pressure, since the entry of the mold for the liquid casting, which should also be the entry of the wire, is not accessible. Indeed, it is placed in close contact with an upper nozzle of a liquid casting ascent shaft or an uphill casting tube coming from the pressurized casting ladle. The entrance of the mold is thus inaccessible to an inoculation wire being unwound. This problem exists both for molds with risers (casting hollow connected with the atmosphere by shafts) and for closed molds (casting hollow without risers, thus without a connection with the atmosphere). Moreover, the technique of low pressure uphill casting does not include risers or a basin upstream from the casting hollow for receiving an inoculant material.